Study On Hot Pressing And Properties Of SiC_nf/SiC Composites Based On Al-B-C Sintering Additives

The liquid phase produced by non-oxide Al-B-C sintering additives can improve the densification degree and high temperature mechanical properties of ceramics,and there is no glass phase residue,which has attracted the attention of researchers.Aiming at the problem that the oxide sintering additives in the sintering process of SiC nanofiber reinforced SiC ceramic matrix composite(SiC_nf/SiC CMC)are easy to oxidize and erode the carbon coated interface layer.In this paper,SiC_nf/SiC composites were prepared by hot-pressing sintering using Al-B-C sintering agent,SiC_nf as reinforcing phase material,and coated with PyC/BN interfacial layer on SiC_nf surface by chemical vapor deposition（CVD）and impregnation cracking method.The effects of sintering aids,SiC_nf content,interface layer,and sintering process on the microstructure and mechanical properties of the composites were studied.The main research contents are as follows:（1）The surface of SiC_nf was coated with PyC/BN interfacial layer by CVD method and impregnation cracking method.PyC was deposited at 1100℃with CH₄ as the carbon source and N₂ as the carrier gas.The effect of deposition time on the coating effect of PyC interlayer was studied.SiC_nf was impregnated in vacuum with a 0.3 mol·L^-1 BN precursor solution,dried,and cleaved at 1000℃to obtain BN interfacial layer.The influence of impregnation times on the morphology of BN interfacial layer was studied.The experimental results show that when CVD deposition time is 60 min,the boundary coating effect is the best.In this case,the deposited PyC interface layer is uniformly coated and compact.The best vacuum impregnation times for BN interfacial layer prepared by impregnation cracking method is 2 times.In this case,the BN interfacial layer thickness of SiC_nf surface is suitable and the coating is more uniform.（2）SiC_nf/SiC composites were prepared by hot press sintering at 1850℃and 30MPa with Al-B-C as sintering agent.The effects of interface layer types on the microstructure and mechanical properties of the composites were studied.SiC_nf was coated with PyC,BN single interface layer,and PyC/BN double interface layer,and then hot-pressed sintering was used to prepare SiC_nf/SiC composite materials with no interface coating and with PyC,BN,and PyC/BN interface coatings,respectively.The results showed that in the composite material without interface coating,the cross-section was relatively flat,and no SiC_nf was observed.When the SiC_nf surface was coated with PyC or BN interface layer,only a small amount of fiber debonding occurred.After the SiC_nfsurface was coated with PyC/BN double interface layer,a significant amount of fiber pull-out was observed.The mechanical property test results showed that the flexural strength of the SiC_nf/SiC composites containing the interface layers of PyC and BN was 351 MPa and 338.5 MPa,and the fracture toughness was 6.98 MPa·m^1/2 and 6.4 MPa·m^1/2,respectively.The toughening effect is not significant.When SiC_nf is coated with PyC/BN double-layer interface,the flexural strength and fracture toughness of the composites prepared by SiC_nf are greatly improved,reaching 414 MPa and 9.4 MPa·m^1/2,respectively,which are 29.38%and 130.39%higher than those of the composite material prepared by SiC_nf coated with no boundary layer.（3）SiC_nf coated with PyC/BN double interfacial layer was mixed with SiC powder and Al-B-C sintering agent,and the SiC_nf/SiC composites were prepared by hot pressing sintering at 1850℃and 30 MPa.The effects of the content of sintering agent and SiC_nfon the microstructure,density and mechanical properties of the composites were studied.The experimental results show that with the increase of sintering aid content,the porosity of SiC_nf/SiC composite material decreased,and the density and relative density increased from 2.375 g/cm³ and 74.3%to 3 g/cm³ and 95.1%,respectively.The mechanical properties of the composite material also improved accordingly.When the content of sintering aids is 10 wt%,the mechanical properties are the most superior,with bending strength and fracture toughness of 414 MPa and 9.65 MPa.With the increase of the amount of SiC_nf,the number of pores in the composite increases,the porosity increases,and the overall density decreases.When the amount of SiC_nf is 8 wt%,the microstructure of SiC_nf/SiC ceramic matrix composite is the densest,and the density reaches the maximum of 3 g/cm³.The mechanical properties of the composites first increase and then decrease with the increase of the amount of SiC_nf.When the amount of SiC_nf added is 10wt%,the mechanical properties are optimal.At this time,the bending strength is 414 MPa,the fracture toughness is 9.65 MPa·m^1/2,and the fracture toughness of the composites is93.8%higher than that of the sintered SiC without SiC_nf.（4）SiC_nf laminated preform was prepared by vacuum filtration,and PyC/BN double interface layer was coated on the SiC_nf preform.SiC_nf/SiC composite material was then prepared by slurry infiltration and hot-press sintering process（SIHP）,and the effects of sintering process（including sintering temperature and pressure）on the comprehensive properties of the composite material were studied.The results showed that compared with the general hot-press sintering process,the samples prepared by SIHP had a higher porosity due to their laminated structure,and the density and relative density decreased from 3 g/cm³ and 95.12%to 2.92 g/cm³ and 92.57%,respectively.However,their flexural strength only decreased from 414 MPa to 390 MPa,while the fracture toughness increased from 9.65 MPa·m^1/2 to 11.02 MPa·m^1/2.The optimum sintering temperature and pressure of SiC_nf/SiC composites prepared by SIHP method are 1900℃and 30 MPa,respectively,at which the comprehensive properties of SiC_nf/SiC CMC were the best,with flexural strength and fracture toughness of 548 MPa and 15.86 MPa·m^1/2,respectively.In addition,the samples at lower temperature and pressure（below 1850℃and 30 MPa）exhibited a pseudo-ductile fracture mode upon failure.In summary,in this paper,SiC_nf/SiC composites were prepared by hot pressing sintering using Al-B-C as the sintering aid and SiC_nf as the reinforcement.The surface of SiC_nf was coated with a PyC/BN double interface layer using chemical vapor deposition（CVD）and immersion pyrolysis methods.The introduction of PyC/BN interfacial layer greatly improves the flexural strength and fracture toughness of the composites.The optimum sintering conditions for SiC_nf/SiC composites prepared by vacuum hot pressing sintering process are as follows:sintering temperature is 1900℃,sintering pressure is 30MPa,and the insulation and pressure preservation time are 1h.Under this process condition,the content of sintering additives and SiC_nf reinforcement are both 10 wt%.The layered structure introduced by SIHP method makes SiC_nf/SiC composites exhibit pseudo-ductile fracture mode at low temperature and pressure,and the bending strength-displacement curve is zigzag.At this stage,the mechanical properties of the composite material have reached their optimum,with a bending strength of 548 MPa and a fracture toughness of 15.86 MPa·m^1/2.This study can provide reference for strengthening,toughening and densification of novel SiC ceramic matrix composites.